Method of machining contact lenses

ABSTRACT

A method for providing a contact lens for a patient. According to this method, an optical preform is selected, and a resting position of the optical preform on the patient&#39;s eye is determined. An optical feature is also located on a patient&#39;s eye. Then, a reference position is located on the surface of the optical preform coincident with the optical feature on the patients eye, when the optical preform is in the resting position. Finally, the optical modification is machined on the optical preform at a location based on the reference position on the optical preform.

This application is a continuation-in-part of Ser. No. 225,386, Apr. 8,1994, U.S. Pat. No. 5,517,259, which is a continuation-in-part of Ser.No. 216,381, Mar. 23, 1994, U.S. Pat. No. 5,592,234 which is acontinuation-in-part of Ser. No. 980,053, Nov. 23, 1992, U.S. Pat. No.5,406,341.

BACKGROUND OF THE INVENTION

The present invention relates generally to methods for manufacturingcontact lenses, and more particularly to methods for manufacturingfinished aspheric single vision contact lenses, or finished spherical oraspheric multifocal contact lenses.

As used herein, the term "multifocal" is used to generally refer tobifocal contact lenses, trifocal contact lenses, progressive contactlenses, and so forth.

While contact lenses are worn by over 10% of all antimetropes in theU.S., multifocal contact lenses have enjoyed only a mixed success. Nomultifocal contact lens has been successfully accepted by more than 70%of the patients fitted with a particular design.

Currently available multifocal contact lenses are designed based on theassumption of an exact fit. An exact fit as defined herein means thatthe contact lens will be centered with respect to the center of thepatient's pupil. In practice, however, since contact lenses are madewith a limited number of concave curvatures, the fit is almost neverexact for an individual patient. Instead, the contact lens positionsitself on the cornea at a position determined by the difference betweenthe curvature of the cornea and the curvature of the contact lens. Therelationship between the lens and the cornea is also affected somewhatby other factors such as lid tension, tear rate and so forth.

So long as the lens is of a single vision type, this accentricitybetween the pupil and contact lens is too small to cause any significantchange in the refractive correction provided by the lens. However,patients wearing other types of lenses such as bifocal contact lensescan suffer a significant loss of visual acuity or contrast due to thisaccentricity. This loss of visual performance of bifocal contact lensescan occur in every bifocal design, although the deleterious effects ofdecentration on visual acuity as a function of contrast may be mitigatedto some extent by centered diffractive bifocal designs. Therefore, lackof perfect fit, which leads to decentration of the lens, is the leadingcause of patient maladaptation to bifocal contact lenses, whether madeof soft hydrophilic materials or of hard gas permeable materials.

One solution to the above problem as recognized by the inventors is toprovide the patient with a lens having a perfect fit. A perfect fit,however, requires a perfect match between the corneal curvature and theconcave curvature of the contact lens. The corneal curvature of eachindividual is unique, and often has zones of pronounced asphericity.Therefore, it is not practical to stock lenses matching all possiblecorneal topographies. Moreover, there are also problems associated withcustomizing the concave surface of each contact lens based on thecorneal curvature, because altering the concave curvature would changethe optical characteristics, i.e., the spherical power or astigmaticcorrection, provided by the lens.

The reason why the add power zone should be within the pupillaryaperture is that for a multifocal lens to function properly, the retinashould receive all the images at the same time. For distant objects, theimage formed by the base power zone is focused, while the image formedby the add power zone is defocused. For near objects, the image formedby the base power zone is defocused, while the image formed by the addpower zone is focused. Given one focused and one or more defocusedimages, the image processing apparatus at the retina and the visualcortex rejects the unfocused images and processes the focused image.

Persons with normal accommodation not requiring any refractivecorrection also receive multiple images simultaneously at their retina,and possess the ability to ignore the defocused image of far objectswhen looking at near objects, and vice versa. This analogy to a normaleye indicates that for a multifocal contact lens to work properly, theadd power zone should be within the pupillary aperture. Since imagestrength at the retina is proportional to the area of the correspondingrefractive zone (i.e., add or base power) subtended at the pupil, theoptimum area of the add power zone can be computed with respect to thepupil size. It is known that pupil size varies from person to person andalso depends on the level of ambient illumination and physicochemicalstatus of the individual. For example, the pupil size of a thirty yearold can vary from 2.2 mm in direct sunlight to 5.7 mm outdoors at night.Data on pupil size distributions by age and illumination level areavailable in the literature. The assumption may also be made that thecontact lens wearer will generally be outdoors when experiencing extremelevels of illumination, where distance vision will be needed the most,whereas ambient illumination is at an intermediate level indoors, wherenear and intermediate vision is required most often. Based on theseconsiderations, it is possible to develop a model which predicts theoptimum sizes of add power zones for near vision, base power zones fordistance vision and aspheric zones for intermediate vision, if needed.Such a model is disclosed in U.S. Pat. No. 5,112,351.

When positioning a multifocal segment on the patients eye, it istypically sufficient to locate the multifocal segment within thepupillary aperture based on the center of the patients pupil. However,it is sometimes desirable to locate the multifocal segment based on theline of sight, or based on some other optical feature of interest. Forexample, although the line of sight of the eye tends to correlate to thecenter of the patients pupil, the line of sight can nonetheless deviatefrom the pupil center in some cases. In cases where the line of sightdeviates from the center of pupil, it is useful to position themultifocal portion of the contact lens with respect to the line ofsight, rather than the center of the pupil.

In other circumstances, it is also useful to position the toric surfaceof a contact lens with respect to the pupillary center or the line ofsight of an astigmatic user. For example, for best adaptation, it ispreferable to orient the axis of the astigmatic correction of toriccontact lens at right angles to the corneal asphericity at the rest orequilibrium position of the lens on the patient's eye. Moreover, it isdesirable that these axes cross at the center of the pupil or at theline of sight, for example.

Thus, in view of the above, there is a need for a contact lens where theadd power zone or toric zone is precisely positioned with respect to anoptical feature associated with the patient's eye, and for a process formaking the same.

SUMMARY OF THE INVENTION

The present invention meets the above needs by providing a novel methodfor manufacturing novel multifocal and/or aspheric contact lenses.According to an embodiment of the present invention, the location of anoptical feature on the patient's eye, such as the center of the pupil orthe line of sight, is identified. An optical preform is also selected,and a position on the surface of the optical preform is determined thatcorresponds to the location of the optical feature when the opticalpreform is stabilized on the corneal surface of the patient's eye.

Then an optical modification is provided on the contact lens, forexample, by machining or by casting. The optical modification can be,for example, a multifocal add power zone or a toric zone or both. Theoptical modification can be cast or machined on the convex surface ofthe optical preform or on the concave surface of the optical preform.

According to an embodiment of the invention, the position on the surfaceof the optical preform corresponding to the optical feature isdetermined by fitting the patient with an optical preform and directlyobserving the position on the surface of the optical preformcorresponding to the optical feature. According to another embodiment,the position on the surface of the optical preform corresponding to theoptical feature is determined by mapping the topography of the patient'scornea. Based on this information the position of the optical preform onthe eye can be predicted. At the same time, the position of the opticalfeature of interest with respect to the eye is determined. Using thisinformation, the position of the optical feature of interest withrespect to the optical preform can be calculated without actuallyfitting the patient with the preform of interest.

In some instances, the rotation of the lens may not stabilize on thecornea to a suitable degree. Lens rotation is undesirable in a number ofcircumstances, including toric lenses and non-centrosymmetric multifocallenses. Thus, it is often desirable to stabilize the contact lens,either prior to placing the lens on the patient or after rotation isobserved on the cornea of the patent. A preferred method of providingrotation stabilization is by means of prism wedges or weights that arecast or machined near the periphery of the optical preform.

According to an embodiment of the invention, the above modifications tothe lens preform are provided by machining the lens preform to form thedistance power, add power and/or toric zone and to form the optionalprism wedges or weights if desired.

Various other embodiments and advantages of the methods of the presentinvention and lenses made thereby will be further evident from thedetailed description of certain embodiments below and from the appendedclaims. The appended claims are hereby incorporated by reference as anenumeration of the above and further preferred embodiments. All patentapplications, patents, and other disclosures referenced in thisspecification are hereby incorporated by reference in their entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of the a spherical, rigid, gas permeable lenssituated on a patient's cornea.

FIG. 2 is a plan view of the lens of FIG. 1, with a diffraction zonepositioned with respect to the center of the patient's pupil.

FIG. 3 is a side view of a contact lens mounted on an arbor formachining.

FIG. 4 is a side view of the contact lens of FIG. 3, with a refractivebifocal zone machined on the lens.

FIG. 5 is a plan view of the lens of FIG. 1 which has been machined toform an astigmatic correction, a bifocal correction and a prism wedge.

DETAILED DESCRIPTION OF THE INVENTION

According to an embodiment of the invention, a contact lens is providedin the following fashion. An appropriate optical preform is selected andthe location of an optical feature (such as the center of the patient'spupil, the patient's line of sight and so forth) is identified withrespect to the patient's eye. A position on the surface of the opticalpreform that corresponds to the optical feature, when the opticalpreform is stabilized on the surface of the patient's eye, is alsodetermined. Then, an optical modification, such as a multifocal zone ora toric region or both is provided on the optical preform, based on theposition on the surface of the optical preform that corresponds to theoptical feature.

If a multifocal zone is to be added, the multifocal zone is preferablydesigned to fit within the pupillary aperture under light conditionscorresponding to conditions where the multifocal characteristics of thelens are to be utilized.

The preform is desirably fabricated from a hydrophilic polymer with lowto high water content or a rigid hydrophobic gas permeable material witha high oxygen permeability (e.g., Dk/1>45). It may also be desirable toform the optical preform from a material that is transparent toultraviolet radiation in the wavelength range of 320-400 nm, providingat least 80% transmission.

The method of the present invention may employ a preform that consistsof a cross-linked, hydrophilic network, with water uptake ranging from37% to 75%, composed of a mixture of acrylates, methacrylates, vinylcarbazoles, at least some of which carry hydroxy or amino substitutes,e.g., hydroxyethyl methacrylate, or N- or C-methyl vinyl carbazole,N,N-dimethylamino ethyl methacrylate, as well as hydrophobic acrylates,methacrylate or vinyl compounds, such as methyl methacrylate, ethylmethacrylate, propyl methacrylate, ethyl acrylate, butyl acrylate,styrene, substituted styrenes and, without limitation, other monomerscommonly used in contact lens manufacturing.

Another type of preform that may be employed is a preform formed from arigid gas permeable material such as a cross-linked siloxane. Thenetwork preferably incorporates appropriate cross-linkers such asN,N'-dimethyl bisacrylamide, ethylene glycol diacrylate, trihydroxypropane triacrylate, pentaerythritol tetraacrylate and other similarpoly-functional acrylates or methacrylates, or vinyl compounds, such asN-methylamino divinyl carbazole.

Thus, while the use of hydrophilic polymers may be recommended in viewof their superior bio-compatibility, the method of fabricating thecontact lenses of the present invention as described herein is fullyapplicable to any other type of optical preform, such as rigid, gaspermeable contact lenses fabricated from acrylic terminated siloxanes orrigid PMMA contact lenses.

If a multifocal zone, such as a bifocal, trifocal or progressive zone isto be provided, then the optical preform can be selected to have nocorrection, distance correction, astigmatic correction, both distanceand astigmatic correction, and so forth. If a toric zone is to beprovided, then the optical preform can be selected to have nocorrection, distance correction, multifocal correction, both distanceand multifocal correction, and so forth. If both astigmatic andmultifocal corrections are to be provided, then the optical preform canbe selected to have no correction, distance correction and so forth.

The spherical power range of commercially available optical preformstypically spans 30 diopters in 0.25 D increments, from +15.00 D to-15.00 D. Commercial preforms may also incorporate cylindricalcorrection in the range of 0.00 D to -5.00 D, in 0.25 D increments. Suchoptical preforms are preferably provided with a mark on opposing ends ofa diameter denoting the direction or the axis of the cylinder, if any isincorporated in the optical preform.

In practicing the method of the present invention, it is desirable todetermine the position on the surface of the cornea where the opticalpreform will stabilize once on the patient's eye. This position can bedetermined, for example, by simply placing the optical preform on thepatient's eye, allowing the contact lens to stabilize and achieve itsnormal resting position, then locating the position of the opticalfeature of interest. If the optical modification to be provided is notcentrosymmetric with respect to the optical preform, then an additionalreference position should be determined. For example, the location ofthe bottom edge of the lens (i.e., the 270° meridian) can be determined.

Although the location of the optical modification can be referenced froma mark placed on the precursor lens surface after it has been fitted asabove, it is also possible to use a custom precursor lens for trialfitting purposes to locate the optical feature of interest, with thecustom precursor lens being of the same curvature as the lens to bedispensed.

Still another way of determining the position of the lens is to use acorneal mapping device to map the surface topography of the patient'scornea. One such device, the Corneal Analysis System model 3, isavailable from EyeSys Technologies, Houston, Tex. Based on this cornealmap, the position of the preform during wear can be predicted. Moreprecise predictions of the position of the optical preform on the corneaduring wear can be obtained by taking into account additional factorssuch as eyelid tension, tear rate of the patient, and the width of thepalpebral opening (i.e., the opening between the eyelids) as well as itsorientation with respect to the cornea.

In addition to determining the position of the optical preform on thepatient's eye, the position of the optical feature of interest isdetermined with respect to the surface of the optical preform. Opticalfeatures of interest include the center of patient's pupil and the lineof sight of the patient's eye (which is within the pupillary aperture,but not necessarily at the center of the pupil).

According to an embodiment of the invention, the position of the opticalfeature, such as the pupil's center, and any other reference positionchosen can then be physically marked on the anterior or convex side ofthe optic. Of course, no physical mark need be made. For example, thelens can be observed through a transparent grid which is used as ayardstick to measure the position of the surface of the optical preformthat corresponds to the optical feature of interest as well as that ofany other reference position chosen (e.g., in cartesian or polarcoordinates).

The patient's line of sight is defined as the line from a fixation point(point of focus) to the center of the entrance pupil and from the centerof the exit pupil to the fovea. Once the line of sight is determined,the point where the line of sight enters the cornea can be determined.The line of sight is a relevant optical axis because it represents thecentral or chief ray of the bundle of light passing from the fixationpoint through the actual optics of the eye to the fovea. Devices areavailable that can be used to determine the location at which the lineof sight enters the cornea. One such device is the Nikon NRK-8000 AutoRef-Keratometer. The patient's vision is first corrected to best visualacuity. Then, using the eccentric fixation mode of this device, thedegree of deviation (e.g., in cartesian coordinants) between fixationand the pupil center is determined. The point of fixation measured bythe device corresponds to the position where the line of sight entersthe cornea. Other methods of determining the line of sight will becomeapparent to those skilled in the art.

In the above embodiments, the center of the center of the pupil or theline of sight was used as a reference point for determining the positionon the surface of the optical preform that corresponds to the opticalfeature of interests. Of course, other reference points are availableand will become immediately apparent to those skilled in the art.Moreover, other embodiments for determining the position of the opticalpreform on the cornea surface and for determining the position of theoptical feature on the optical preform will become apparent to thoseskilled in the art.

Once the position of the optical feature is known with respect to thesurface of the optical preform, then the lens is provided with theappropriate optical modification, which can be, for example, a distancezone, a toric zone, an add power zone and/or a stabilizing zone. If amultifocal add power zone is to be employed, either a bifocal-style addzone (e.g., a diffraction bifocal zone, a spherical crescent or flat topconfiguration) or a progressive-addition-style add power zone may beselected depending on the patient's desires and lifestyle. Preferredmethods for providing the optical modification include casting andmachining.

When appropriate, any of the above modified optical performs can bestabilized against rotation. For example, as noted above, the fittedlens may not stabilize on the cornea of the patient to a suitabledegree. In such cases, it may be desirable to stabilize the contactlens, either prior to placing the lens on the patient or after rotationis observed on the cornea of the patent. Observation of rotation on thepatient's cornea can be enhanced by means of the mark that is optionallyused to designate the location of an optical feature with respect to thepreform. Methods of stabilizing contact lenses are known in the art andinclude weighting, truncation, prism balancing, toric design on thefront or back of the lens, and so forth. Stabilization can also beachieved by matching closely the topography of the cornea with theconcave surface topography of the contact lens.

According to an embodiment of the invention, the optical modifications(for example, an add power zone and/or a toric zone) and therotation-stabilizing features are machined into the optical preform atthe desired location. Conventional machining methods may be employed toform the modifications of the optical preform.

For example, rigid gas permeable lenses can be machined on a precisionCNC lathe or a contact lens milling machine. The optical preform may beblocked onto a precision arbor with a suitable wax which is then mountedin position by a collet on the chuck of the lathe. Alternatively, thelens can be held in place by a vacuum. The location of the center of thelens can serve as a positional guide for the machining process.

Soft hydrophilic contact lenses can also be machined. For example, softcontact lenses can be machined at room temperature at a partiallydehydrated state. Or they can be machined after being frozen. Machiningof frozen lenses is a well established technology. The frozen lenses canbe either partially dehydrated or machined in the fully hydrated state.To carry out the machining process, the lenses are mounted on a collet(for example, with double sided adhesive tape). Then the soft lenses areexposed to a stream of cold, dry nitrogen gas. After the lens freezes,the flow of nitrogen is maintained at a reduced level that isnonetheless adequate to remove the heat generated by the machineprocess. By maintaining the lens at a constant low temperature, opticaldistortions in the lenses are minimized.

Referring now to the drawings, FIG. 1 shows a plan view of a spherical,rigid, gas permeable lens 20 situated on a patient's cornea over thepupil 10. The location of the pupillary center 11 and the 270° meridian23 have been marked on the convex surface of the lens. Numeral 26designates the geometrical center of the lens, and numeral 12 designatesthe line of sight. FIG. 2 is a plan view of the lens 20 of FIG. 1 aftera diffraction bifocal add zone 21 has been machined into the convexsurface of the lens. The add zone 21 is centered on the point 12 markingthe location of the line of sight of FIG. 1.

FIG. 3 represents a side view of a soft contact lens 20 mounted on anarbor 30 by means of a double sided adhesive tape 40. The curvature ofthe surface of the arbor 30 is substantially the same as the curvatureof the concave surface of the contact lens 20 to avoid any buckling ordistortion of the lens 20 as it is cooled down. FIG. 4 is a side view ofthe lens 20 shown in FIG. 3, mounted as in FIG. 3 with a refractivebifocal zone 21 machined into the convex surface of the lens 20. Thepresence of a prism wedge 22 can be seen at the 270° meridian.

FIG. 5 shows a plan view of the rigid gas permeable lens 20 of FIG. 1which has been machined to form a cylindrical correction at the 45°-135°meridian, such that the cylinder correction axis bisects the pupillaryaperture as projected on the lens surface. A bifocal segment 21 andprism wedge 22 have also been machined.

The above has been a detailed discussion of certain embodiments of thepresent invention. They should not be considered so as to limit thescope of applicants' invention which is defined by the appended claims.

What is claimed is:
 1. A method for providing a contact lens for apatient comprising:locating an optical feature on a patient's eye;selecting an optical preform; determining a resting position of saidoptical preform when said optical preform is stabilized on the patient'seye to locate a reference position on the surface of the optical preformcoincident with the optical feature on the patients eye; and machiningan optical modification on said optical preform at a location based onthe reference position on the optical preform.
 2. The method of claim 1,wherein said contact lens is a single vision contact lens with amultifocal add power zone.
 3. The method of claim 1, wherein saidcontact lens is a single vision lens with a toric zone.
 4. The method ofclaim 1, wherein said contact lens is a toric or astigmatic lens with amultifocal add power zone.
 5. The method of claim 1, wherein a mass ismachined from the optical preform to stabilize the optical preformagainst rotation.
 6. The method of claim 1, wherein said optical featureis the center of the pupil of the patient's eye.
 7. The method of claim1, wherein said optical feature is the line of sight of the patient'seye.
 8. The method of claim 1, wherein said step of determining the restposition of the optical preform comprises:fitting a patient with anoptical preform; and observing the position on the surface of theoptical preform corresponding to the optical feature.
 9. The method ofclaim 1, wherein said step of determining a rest position of the opticalpreform is determined without a fitting a trial lens on the patient'seye.
 10. The method of claim 1, wherein the convex side of the opticalpreform is machined.
 11. The method of claim 1, wherein the concave sideof the optical preform is machined.
 12. A contact lens having an opticalmodification provided by machining, said optical modification beinglocated based on a position of an optical feature on the patients eyerelative to the contact lens when the lens is stabilized on thepatient's eye.
 13. The contact lens of claim 12, wherein said contactlens is single vision contact lens with a multifocal add power zone. 14.The contact lens of claim 12, wherein said contact lens is a singlevision lens with a toric zone.
 15. The contact lens of claim 12, whereinsaid contact lens is a toric or astigmatic lens with a multifocal addpower zone.
 16. The contact lens of claim 12, wherein a stabilizingfeature has been machined into the contact lens.
 17. The method of claim12, wherein said optical feature is machined on the convex side of thecontact lens.
 18. The method of claim 12, wherein said optical featureis machined on the concave side of the contact lens.
 19. The contactlens of claim 12, wherein said optical feature is the center of thepupil of the patient's eye.
 20. The contact lens of claim 12, whereinsaid optical feature is the line of sight of the patient's eye.